Rapid, simple and reliable detection of certain analytes in a sample is of crucial importance in various fields of technology and natural sciences. Numerous detection methods are employed, depending on the requirements and on the desired accuracy. A method known from the prior art, and existing in many different variations, is of an optical nature and is based on the fact that the analyte to be detected in many cases exerts an influence on the optical properties of certain indicator substances. Thus, for example, the analyte can influence a fluorescence and/or phosphorescence behavior of an indicator substance, for example by reducing (quenching) the intensity of a fluorescence light, which the indicator substance emits under excitation by an excitation light source, in the presence of the analyte to be detected. Instead of a reduction, the opposite effect can also be exploited, for example by a fluorescence behavior being increased, or even being initially generated, when the analyte interacts with the indicator substance. Many such methods are known from the prior art.
Thus, for example, in B. Choudhury et al.: Glucose biosensors based on organic light-emitting devices structurally integrated with a luminescent sensing element, Journal of Applied Physics, 96 (5), 2949, 2004, a device is described which uses an optical method for glucose detection, said method being based on enzymatic oxidation of glucose in the presence of glucose oxidase (GOD). The glucose concentration is measured by means of an oxygen-sensitive dye, which is present together with the glucose oxidase in an indicator layer or in a solution. Instead of glucose oxidase for detection of glucose, it is alternatively possible to use glucose dehydrogenase (GlucDH) as catalyst for converting glucose to gluconolactone in the presence of NAD (see, for example, the illustrative embodiments in DE 103 04 448 A1 or WO 03/097859 A2).
In addition to the detection of glucose, other types of detection of various analytes are known, especially from the field of medicine. For example, in O. Hofmann et al.: Towards microalbuminuria determination on a disposable diagnostic microchip with integrated fluorescence detection based on thin-film organic light-emitting diodes, Lab Chip, 2005, 5, 863-868, a method is described for detection of HSA (human serum albumin). The method is based on a reaction of HSA with the dye albumin blue 580, which generates a strong emission at 620 nm when excitation takes place with a light of suitable wavelength.
Other chemical and/or biochemical forms of optical detection are also known from the prior art. For example, U.S. Pat. No. 6,331,438 B1 discloses a series of dyes that can be used for detecting molecular oxygen. Other types of analytes can also be detected directly or indirectly in this way.
Particularly in the area of mobile chemical analysis, environmental analysis or medical diagnostics, however, a problem is that conventional optical detection methods, which require complex apparatus (e.g. large-volume light sources with monochromators, lasers or photometers), are not really practicable, particularly for portable use. Therefore, in the prior art, various attempts have been made to make available test devices or test elements based on partial integration of one of the described optical detection methods. For example, EP 0 811 154 B1 discloses an optical fluorescence sensor based on an indicator molecule which is embedded in a polymer matrix. For the fluorescence detection, a photodetector, an excitation light-emitting diode and an indicator membrane are integrated as components into the fluorescence sensor.
EP 0 244 394 B1 discloses a microstructured sensor element which is used for determining substance concentrations in gaseous and liquid samples and which has a carrier layer and an indicator layer. A substrate, in which at least one photosensitive element is integrated, is arranged on the carrier layer. This substrate has at least one area that is permeable to the excitation radiation for excitation of the indicator substance.
U.S. Pat. No. 4,889,690 discloses a sensor arrangement which is used to measure physical parameters or concentrations of particles and which comprises a laminar light source. Indicator particles that fluoresce under excitation by the laminar light source are embedded in an indicator layer, the fluorescence light being detected by a photoelectric receiver.
U.S. Pat. No. 6,331,438 B1 discloses an optical sensor used for detecting chemical, biological or physical analytes. Here, an analyte-sensitive layer is optically coupled to a thin-film electroluminescence layer. The optical response of the analyte-sensitive layer to the excitation light is picked up by a photodetector.
From the article by B. Choudhury et al., already cited above, an arrangement for glucose detection is known which is based on excitation of a dye that emits in the red spectral range and is embedded together with glucose oxidase in an indicator layer. This arrangement uses what is called a “reverse face detection method” in which the indicator layer is excited through a glass substrate by means of an organic light-emitting diode (OLED). The fluorescence light is measured by means of a photomultiplier (PMT), also after passing through the glass substrate.
A detection device for detecting HSA is known from the already cited article by O. Hofmann et al. There, a microstructured chip is used in which a liquid sample containing the analyte is conveyed into a detection chamber. Excitation takes place there by means of an organic light-emitting diode, and fluorescence light is conveyed to a spectrometer by means of a spectrometer fiber and is analyzed.
However, the devices and methods known from the prior art have numerous disadvantages in terms of practical use, in particular in terms of their use in portable medical diagnostics, and it is these disadvantages that have hitherto prevented widespread use in chemical analysis or medical diagnostics. For example, many of the disclosed methods are associated with extremely complex assembly of components, such as the sensor element disclosed in EP 0 811 154 B1. Because of their high production costs, such sensor elements cannot really be used for one-off tests in particular. Even in cases where it is only individual parts of such sensors that have to be replaced, the complex replacement procedure prevents use in portable medical diagnostics, where elderly patients and children, for example, have to operate such devices. The sensor element known from EP 0 244 394 B1, and involving complex inorganic microstructuring methods such as lithography techniques or the like, is also not really practicable for many areas of medical diagnostics, because of its complexity and because of the high production costs associated with the microstructuring methods.
The devices known from U.S. Pat. No. 4,889,690, from U.S. Pat. No. 6,331,438 B1, from the aforementioned publication by B. Choudhury et al., and also from the above-mentioned publication by O. Hofmann et al, are also associated with various disadvantages, which in particular relate to the use of elaborate detection devices for the fluorescence light. For example, in the devices used by B. Choudhury et al. and O. Hofmann et al., a large-volume photomultiplier or even a spectrometer and fiber system are necessary, which largely rules out their use in portable medical diagnostics. Similarly complex is the photodetector disclosed in U.S. Pat. No. 6,331,438 B1, which is arranged separately from the actual test device and can be used for a time-resolution method, and which must therefore have a correspondingly complex design. The sensor element disclosed in U.S. Pat. No. 4,889,690, and based on the use of electro-chemoluminescent radiation sources, requires high operating voltages and at the same time provides low light output, thus greatly increasing the demands placed on the photodetector used.
A further disadvantage of the devices disclosed in U.S. Pat. No. 6,331,438 and in the publication by B. Choudhury et al. is that the layer structures shown entail a glass substrate being coated on both faces. Such devices coated on both faces are difficult to produce in practice, particularly since the application of a metal layer on one face of the substrate at the same time leads to contamination of the reverse face of the substrate, which cannot even be avoided by complex protective measures. Also, handling of sensitive substrates coated on both faces, in particular an inexpensive automated handling, is difficult to achieve in practice. Moreover, test elements coated on both faces are more difficult for a patient to use.